1. Field of the Invention(s)
The invention(s) relates to a semiconductor test system and method. More particularly, the invention(s) relates to a semiconductor test system and method that improves probe head to wafer contact using dynamic overdrive compensation.
2. Description of the Related Art
Semiconductor devices, e.g., wafers and integrated circuits, are typically tested at various points in their manufacture. This includes the wafer-test step, where the circuit units are tested while they are still part of a wafer and have not been packaged into individual units. Testing ensures the devices—and the manufacturing processes used to produce them—are working properly and within acceptable quality ranges.
A typical wafer-test is performed as follows. The wafer is placed on a chuck or similar mechanical tool that moves the wafer and brings the device under test (DUT) into contact with an interface unit, e.g., a sort interface unit (SIU). The SIU provides electrical signals received from a tester to the wafer through contact pins positioned on a probe head (which is a part of the SIU). The contact pins are often spring-loaded.
In some cases, the probe head flexes or bends due to the force from the chuck, resulting in poor electrical contact. Poor or inconsistent electrical contact can also result from contact pin non-planarity. That is, some pins are shorter than others such that the shorter pins do not connect to the wafer. Wafer-to-wafer variations can also contribute to poor electrical contact.
Other factors such as airflow within the prober and manufacturing floor shakes might cause enough pin movement to prevent reliable contact with the wafer. Poor or inconsistent electrical contact increases test failures, lowering yield and throughput.
One method of improving probe to wafer contact is to use Optical Z Alignment (OZA) followed by aggressive overdrive. In OZA, the chuck moves the wafer in X, Y, and Z directions to optically align it with the probe head's contact. Then, the chuck is significantly overdriven in the Z direction such that the wafer is brought into pin contact with increased force, whether necessary or not. This aggressive overdrive might cause long-term damage to the SIU and, more particularly, to the probe head. The overdrive amount is typically chosen to work with most SIUs and wafer types. As a result, a limitation of the OZA method followed by aggressive overdrive is that it does not account for SIU-to-SIU and wafer-to-wafer variations. The amount of overdrive, therefore, is chosen aggressively to envelope most SIUs and wafer lots. And the amount of overdrive is chosen without specific knowledge of the particular SIU used and the particular wafer being tested. The result is a very high overdrive that might cause long-term damage to the SIU or wafer.
Electrical Z Alignment (EZA) addresses some of the disadvantages associated with OZA. In EZA, the chuck moves the wafer closer to the SIU probe head until a bare electrical contact is established on all pins. A bare contact is established on all channels when any current flows through the pins. Once a bare contact is established on all pins, the test system overdrives the chuck to account for wafer-to-wafer variations, much like in the OZA method we describe above. Here, however, the overdrive used in the EZA method is not as aggressive or significant as it is in the OZA method. In both methods, however, the amount of overdrive is determined heuristically and its accuracy cannot be guaranteed until the product is mature.
If a contact between the probe head and the wafer is not reliable, the wafers often need to be re-tested, increasing test time and consequently, test cost.